The invention is the use of a particular polyamide polymer material called “microcrystalline” for the purpose of obtaining decorative and functional objects having aesthetic, attractive and high-quality visual and tactile properties.
It is also desirable for these visuo-tactile properties to be lasting when faced with mechanical (impact, scratching), chemical (solvent) and physical (UV) attack. Typically the manufacture of the object comprises steps carried out with the material hot, in particular between the Tg (glass transition temperature) and the Tm (melting point) of this microcrystalline polyamide. Typically, the use of the object (subsequent life of the finished object) will be at a temperature below the Tg of this microcrystalline polyamide.
Among polymer materials, amorphous polymers have the advantage of being transparent. Besides this intrinsic aesthetic advantage, they make it possible to protect and to bring out an underlying decoration. Among these amorphous polymers, mention may be made of PMMA, PC and amorphous PAs. The latter are of particularly high performance (EP 550 308 and EP 725 101). However, while they are being processed in the melt, they have the drawback of rapidly going into the solid state (owing to their high Tg, namely 100-200° C.) as they are being cooled and are therefore ill-suited for faithfully retranscribing the surface finish and feel of the mould and, more generally, of a complex texturizing surface. Since they are typically very rigid and barely malleable below their Tg, they are ill-suited to being formed in the solid state (for example by stamping). An amorphous polymer of low Tg (<60° C.) is itself barely able to be envisaged, as it passes into the liquid state above its Tg, which of course makes it unsuitable for fulfilling its role of protecting the decorated object whenever the temperature rises somewhat. Another drawback of amorphous polymers, and even of amorphous PAs based on high-carbon monomers (e.g.: PA-BMACM.1/12), is the inferior chemical resistance (to stress cracking) and physical resistance (to UV radiation) compared with semicrystalline polymers, especially semicrystalline polyamides based on high-carbon monomers such as PA-11 or PA-12.
Among polymer materials, semicrystalline polymers therefore have the advantage of better chemical and physical resistance. Among these, semicrystalline polyamides constitute an advantageous choice. Among semicrystalline polyamides, preferred ones are those made from high-carbon monomers, such as PA-11 and PA-12, since their physico-chemical resistance is even better, and their water uptake and the consequences in terms of dimensional variations (and variations in other properties) are less than in the case of standard semicrystalline polyamides such as PA-6 and PA-6,6. However, these semicrystalline polyamides have the disadvantage of having a limited transparency and of passing rapidly into the solid state (owing to their fast and high recrystallization rate) while they cool, and are therefore ill-suited for faithfully retranscribing the surface finish and feel of the mould.
We have discovered that the use of a particular polymer, namely a “microcrystalline” polyamide, in other words a transparent but nevertheless semicrystalline polyamide with a particular degree of crystallinity, can provide a particularly advantageous solution for obtaining decorative and functional objects having aesthetic, attractive and high-quality visuo-tactile properties. The polyamides used in the invention are those from semicrystalline polyamides that are microcrystalline, that is to say those consisting of crystalline structures (spherulites) having a size small enough not to diffract light and thus allowing good transparency. In the rest of the text, these will be referred to as “microcrystallines”. They may also be characterized by a transparency such that the light transmission at 560 nm on a polished object 1 mm in thickness is greater than 80%, advantageously greater than 88% (the object being obtained by standard processing methods, such as injection moulding and sheet extrusion/calendering).
This microcrystalline polyamide has many advantages. This is because such a material does not have the drawbacks of:                low transparency;        solidifying too rapidly;        passing into the liquid state above its Tg;        having a mediocre mechanical impact and scratch resistance;        having a mediocre chemical and stress-cracking resistance; and        having a mediocre UV resistance.        
In fact, such a material has the key advantage of being easily formed by solid-state (or partly solid-state) forming between its Tg and its Tm, thanks to its malleability in this temperature range. The expression “solid-state (or partly solid-state) forming” is understood to mean various “warm” or “hot” thermomechanical treatments between Tg and Tm, for the purpose of giving a finish possessing an aesthetic, attractive and high-quality and visuo-tactile character to the polymer material (and to the object of which this polymer material is one of the constituents). We mention by way of examples of such solid-state forming the following:                passage from a 2D (two-dimensional) form, for example a 600 μm sheet of the polymer material, to a 3D (three-dimensional) form has the result of a step using a thermoforming or stamping process between Tg and Tm;        passage from one surface finish to another (smooth to rough), typically by a step and a process of bringing the material into contact with a textured surface (for example a rough metal or a fabric), by compression moulding or overmoulding, between Tg and Tm, under pressure, for a certain time;        passage from a small-sized form (powder, small tile, sheet of small area) to a larger form (bulk object, tiled surface), typically by a sintering or welding process, between Tg and Tm, under pressure, for a certain time;        complexing, lamination, or assembling, for example of a 600 μm sheet onto a substrate possessing anfractuosities (wood, fabric), for example during a step of a coating or lamination process;        complexing or transfer, for example onto a 600 μm sheet of the polymer material, of fibrils or powder (whether pigmented or not) for example during a step of a transfer process. This process consists, for example, in bringing into contact, at a temperature T between Tg and Tm, under a pressure P, for a time t, a sheet of polymer material with a substrate containing the fibrils (e.g. a fabric), the said fibrils being transferred from the substrate to the polymer material in which they will become mechanically (and even also chemically) anchored, thereby giving the material a particularly soft and warm feel. Another example is that in which the polymer sheet is brought into contact with a bed of polymer powder (e.g. PA-11) under similar T, P, t conditions, all this giving us a material with a powder feel;        superior mechanical resistance to impacts, knocks and scratches, which resistance is most particularly manifested in terms of little visual impact of the attack (no fraying, bleaching, etc.) and not only in terms of weight loss or energy value;        hardness and non-malleability at Tambient and at T<Tg;        complete transparency, typically greater than or equal to that of a conventional amorphous polymer such as polycarbonate (PC), this being so for identical thicknesses of less than 2 mm;        chemical and stress-cracking resistance comparable to a semicrystalline PA (e.g. PA-11);        excellent UV resistance; and        possibility of being decorated by sublimation (in addition to more conventional techniques such as screen printing).        